1. Field of the Invention
The present invention relates to an image forming apparatus adapted to transfer a toner image carried by an image carrier onto a sheet, and a density control method for the image forming apparatus.
2. Description of the Related Art
Conventionally, in an image forming apparatus, such as a copying machine or a printer utilizing electrophotography, the density of an image has been corrected in the following manner: A toner image is formed for density correction (i.e. a toner patch) on an image carrier, such as a photosensitive drum or an intermediate transfer member, and the toner patch is detected by an optical sensor to thereby generate correction data.
In the case of determining the density of a toner patch, it is required to grasp the amount of reflected light from a portion of the image carrier reflected light from a portion of the image carrier where the toner patch is to be formed, i.e. from a so-called background, in advance. This is because an output from the sensor having detected the toner patch also contains reflected light from the background.
The photosensitive drum and the intermediate transfer member are glossy, so that much of light irradiated onto the photosensitive drum or the intermediate transfer member is reflected therefrom to be read by the optical sensor. In particular, a low-density image is expressed by reducing the amount of toner, and hence the degree of exposure of a background on which a low-density toner patch is formed is higher than that of a background on which a high-density toner patch is formed. For this reason, in order to calculate the density of a low-density toner patch accurately, it is required to detect the density of the toner patch while taking into account the amount of reflected light from its background.
Conventionally, there has been proposed a method in which a home position mark provided on an image carrier is detected by an optical sensor to thereby obtain the positional relationship between the home position of the image carrier for a rotation thereof and a toner patch, and then the amount of reflected light from the background of the toner patch is identified based on the positional relationship (see Japanese Laid-Open Patent Publication No. 2005-345740).
In this method, surface conditions of the image carrier during one rotation of the same are detected as a profile in advance. Further, an output indicative of reflected light from the background of the toner patch is identified based on the positional relationship between the home position and the toner patch and the profile of the surface conditions of the image carrier detected in advance over one rotation of the image carrier, and the density of the toner patch is detected based on the identified output indicative of reflected light from the background and the result of detection of the toner patch.
Further, conventionally, there has been proposed an apparatus which does not use the above-mentioned home position mark (see Japanese Laid-Open Patent Publication No. 2005-148299). In this apparatus, background data corresponding to one rotation of an intermediate transfer member is measured, and then image density detection data corresponding to one rotation of the intermediate transfer member having a toner patch formed thereon is measured. Thereafter, alignment between the background data and the image density detection data is performed based on a correlation between the two data. Thus, background data on a portion of the intermediate transfer member where the toner patch is formed is identified based on the result of the alignment.
However, the conventional image forming apparatuses described above suffer from the following problems: In the image forming apparatus disclosed in Japanese Laid-Open Patent Publication No. 2005-345740, if the home position mark is lost due to fall-off or abrasion, it becomes impossible to perform density correction by taking reflected light from the background into account. Further, it takes cost to attach the home position mark.
On the other hand, in the image forming apparatus disclosed in Japanese Laid-Open Patent Publication No. 2005-148299, after acquisition of the background data, it is required to cause the intermediate transfer member to perform one more rotation with the toner patch formed thereon, so as to acquire data for density correction, and therefore it takes time to perform density correction.